An anti-flu antiviral
Oseltamivir
An antiviral medicine that treats and helps prevent flu by stopping the flu virus spreading in the body.
What is Oseltamivir?
Oseltamivir is an antiviral medicine used to treat and sometimes prevent flu (influenza). It works best when started within about two days of symptoms beginning, and it can shorten how long flu symptoms last, and in higher-risk people may reduce the chance of the illness becoming more severe. It is not a substitute for the yearly flu vaccine, and nausea is its most common side effect.
Education and reference only. This is a plain-language guide to Oseltamivir — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Oseltamivir is the main antiviral used in the UK to treat flu (influenza) and, in some situations, to help prevent it after close contact with someone who has flu. It is used particularly in people at higher risk of complications — such as older people, pregnant women, and those with long-term heart, lung or other conditions — and during outbreaks. It is taken as a capsule or as a liquid for those who cannot swallow capsules, usually as a short course.
How it works
Flu viruses copy themselves inside the body's cells and then need to break free to infect new cells. Oseltamivir blocks an enzyme on the surface of the flu virus called neuraminidase, which the virus uses to release newly made copies. By stopping this release, oseltamivir slows the spread of the virus through the body, which helps the illness settle sooner and, in higher-risk people, may reduce the chance of the illness becoming more severe. Because it acts on the virus's own spreading mechanism, it works best when started early before the virus has multiplied widely.
Company & origin
Originated / developed by: Developed by Gilead Sciences and marketed by Roche..
Introduced around 1999, oseltamivir is the most widely used antiviral for influenza in the UK.
Practical use
How to take Oseltamivir
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- Start it as soon as possible, ideally within about two days of symptoms beginning.
- Take it with or without food; taking it with food can ease nausea.
- Finish the full course even if you start to feel better.
- Use the liquid form if you cannot swallow capsules, measuring it as directed.
- If a dose is missed, take it when you remember unless the next is near, then skip it — do not double up.
Weighing it up
Advantages & disadvantages of Oseltamivir
Advantages
- Can shorten how long flu symptoms last, and in higher-risk people may reduce the chance of the illness becoming more severe.
- Can also be used to help prevent flu after close contact with a case.
- Available as a capsule and a liquid, and given as a simple short course.
Disadvantages
- Only useful for flu, and works best when started very early after symptoms begin.
- Commonly causes nausea and sometimes vomiting.
- Not a substitute for the annual flu vaccine, which is the main preventive measure.
Practical use
Good to know
Timing is everything: oseltamivir works best when started within about two days of the first flu symptoms, so seek advice quickly if you are at higher risk. It can be taken with or without food, but taking it with food can reduce nausea, the most common side effect. It is a short course, and the full course should be finished even if you feel better. Importantly, it is not a replacement for the annual flu vaccine, which remains the main way to prevent flu.
Who should not take it / use with caution
- People who have had a serious allergic reaction to oseltamivir.
- The amount may need adjusting in significant kidney problems — tell your prescriber.
- Used with care in pregnancy and breastfeeding, weighing the benefits against the risks with your clinician.
Monitoring
- Watching that flu symptoms are improving
- Checking for nausea or vomiting affecting the course
- Kidney function may be considered before dosing in those with kidney problems
Side effects
- Nausea and vomiting, which are less likely if taken with food.
- Headache and stomach ache.
- Rarely, skin reactions or, particularly in children and teenagers, unusual behaviour or confusion — seek advice if this occurs.
Key interactions
- Few important interactions; it is generally well tolerated alongside other medicines.
- The live (nasal spray) flu vaccine may be less effective if given around the same time as oseltamivir.
- Tell your prescriber about all your medicines, including any over-the-counter remedies.
Available as: Capsules and an oral liquid (a liquid can also be made up from the capsules).
Answers
Oseltamivir: frequently asked questions
When should I start oseltamivir?
As early as possible — ideally within about two days of your first flu symptoms. It works by slowing the virus before it spreads widely, so starting late reduces the benefit. If you are at higher risk of complications, seek advice promptly.
Does oseltamivir replace the flu jab?
No. The annual flu vaccine is the main way to prevent flu and is recommended for eligible groups every year. Oseltamivir is used to treat flu or to prevent it after close contact, not as a substitute for vaccination.
It is making me feel sick — what can I do?
Nausea is the most common side effect and is usually milder if you take each dose with food. If vomiting is severe or you cannot keep the medicine down, contact your prescriber for advice.
What is the difference between oseltamivir and Tamiflu?
They are the same medicine. Oseltamivir is the generic name and Tamiflu is a brand name; both contain the same active ingredient.
Can children take oseltamivir?
Yes, it can be used in children, including as a liquid, when flu treatment or prevention is needed. Carers should seek advice if a child shows any unusual behaviour or confusion while taking it.
The wider class
About Influenza antivirals
Oseltamivir belongs to the influenza antivirals class. For how the class as a whole works, its shared safety principles and monitoring, see the full guide.
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Authoritative sources
- BNF: Oseltamivir.
- NICE CKS: Antiviral medicines for flu.
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